This invention relates to ceramic block filters with high performance in a small package.
A ceramic body with a coaxial hole bored through its length forms a resonator that resonates at a specific frequency determined by the length of the hole and the effective dielectric constant of the ceramic material. The holes are typically circular, or elliptical. A dielectric ceramic filter is formed by combining multiple resonators. The holes in a filter must pass through the entire block, from the top surface to the bottom surface. This means that the depth of hole is the exact same length as the axial length of a filter. The axial length of a filter is set based on the desired frequency and available dielectric constant of the ceramic.
The ceramic block functions as a filter because the resonators are coupled inductively and/or capacitively between every two adjacent resonators. These components are formed by the electrode pattern which is designed on the top surface of the ceramic block couplings and plated with a conductive material such as silver or copper.
Ceramic filters are well known in the art and are generally described for example in U.S. Pat. Nos. 4,431,977; 5,250,916; and 5,488,335, all of which are hereby incorporated by reference as if fully set forth herein.
With respect to its performance, it is known in the art that the band pass characteristics of a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are increased. The number of holes required depends on the desirable attenuation properties of the filter. Typically a simplex filter requires at least two holes and a duplexer needs more than three holes. This is illustrated in FIG. 1 where graph 10 represents the filter response with fewer holes than graphs 12 and 14. It is apparent that graph 14 which is the response of the filter with the most holes, is the sharpest of the three responses shown. Referring to FIG. 2, it can be seen that the band pass characteristic of a particular dielectric ceramic filter is also sharpened with the use of trap holes bored into the ceramic block. Solid line graph 21 represents the response of a filter without a high end trap. Dashed line graph 23 represents the response of the same filter with a high end trap.
Trap holes, or traps as they are commonly referred to are resonators which resonate at a frequency different from the primary filter holes, commonly referred to simply as holes. They are designed to resonate at the undesirable frequencies. Thus, the holes transmit signals at the desirable frequencies while the traps remove signals at the undesirable frequencies, whether low end or high end. In this manner the characteristic of the filter is defined, i.e. high pass, low pass, or band pass. The traps are spaced from holes a distance greater than the spacing between holes so as to avoid mutual interference between the holes and traps. As shown in FIG. 3, whereas holes 31 are separated from each other a distance equal to D, a distance of 2D is placed between trap 33 and the transmission hole nearest to trap 33. The precise distance between trap and transmission pole is one of design choice for achieving a specified performance, but it is preferably 1 to 10 mm. Traditionally, the traps will be spaced from 1.5D to 2D from the holes.
Conventionally the holes 41 and traps 43 in a ceramic filter are positioned along a straight line, as shown in FIG. 4. This design together with the spacing requirements addressed above limits the extent to which a filter may be reduced in size. Specifically, the performance characteristics of a given filter are a function of its width, length, number of holes and diameter of holes. The usual axial length L is 2 to 20 mm. The width w is determined by the number of holes. The usual width of the block filter is 2 to 70 mm. Reducing the number of holes, the diameter of the holes, or the spacing between holes, will effect the performance. Accordingly, it is desirable to have a design for a dielectric ceramic filter which can effectively reduce the size of a given filter while maintaining its given performance characteristics.
A new design for reducing the size of a given filter is achieved by reducing the diameter of the traps and moving them off center from the transmission holes while shortening the distance between the trap and the next nearest transmission hole. Thus the width of any given filter is reduced without effecting the performance of the filter. In one specific embodiment of the present invention, each trap is positioned from the next nearest transmission pole a horizontal distance of 0.8D to 1.5D, where D is the spacing between transmission poles. Each trap is also vertically spaced from the transmission poles a distance between 1/8H and 3/8H, where H is the height of the filter.